Vtyjkyuk

You don't need to know any differential geometry to grasp the basic ideas in Lie theory beyond some idea of what a tangent vector is. The study of semisimple Lie groups (which includes $E_8$) is largely algebraic (there are theorems that make this precise but you don't need to know what they are) and getting a good grasp of the important examples doesn't require more than comfort with calculus and linear algebra.

I would recommend Stillwell's Naive Lie Theory in this vein. I agree with Matt E that Fulton and Harris is also a solid resource.

One of the main points of interest with regard to Lie group is their representations, and I think studying them together with their representations
makes a lot of sense.

To this end, I recommend Fulton and Harris's book on representation theory. About 3/4 of it is devoted to Lie groups, and it light on the theoretical background (although it does presume some mathematical maturity) and heavy on examples and intuition.

Personally, seeing as you are a high school student, I would start out a little lighter--even lighter than Brian C. Hall's book.

I think the perfect place to get a painless introduction to Lie theory, that gives you the exact right idea without all of the necessary machinery is the little gem of a book "Matrix Groups for Undergraduates" by Tapp. There you will be introduced, in a very congenial and pleasant way, to Lie groups and the ideas of differential geometry simultaneously.

Once you get used to that I would suggest the book by Brian C. Hall that others have mentioned as well as the books by Sepanski and Tom Dieck. In fact, these are the recommended books for the Lie groups part of a course on Lie Groups/Algebraic Groups I'm taking with Jeffrey Adams (one of the big players in the discovery the article you linked to mentioned). You should get a good feel for compact Lie groups before you move onto the more advanced methods needed to discuss non-compact Lie groups.

Also, the notes by Ban and the accompanying lectures are great once you feel prepared to learn about non-compact Lie groups.

Also, an absolutely must read, for when you start learning the more advanced (i.e. anything beyond Tapp's book) topics in Lie groups is the fantastic introductory article Very Basic Lie Theory by Howe.

Lie groups are groups (obviously), but they are also smooth manifolds. Therefore, they usually come up in that context. If you want to learn about Lie groups, I recommend Daniel Bump's Lie groups and Anthony Knapp's Lie groups beyond an Introduction. But be aware that you need to know about smooth manifolds before delving into this topic; knowledge of basic group theory is not enough.

Also, as Adam Saltz noted boelow in the comments, if you want a book that treats both smooth manifolds and Lie groups, you can look at John Lee's Introduction to Smooth manifolds

An introductory book in abstract algebra (at the same level of Dummit Foote) that does discuss the basic ideas of Lie Algebras (in a beautiful and not too technical way) is Michael Artin's Algebra.

Check it out!

I'd say Chevalley's book "Theory of Lie Groups I" is a good reference. I'm currently using him (yes, I'm studying Lie Groups too!). Take a look at it and see if it is what you need.

There is a modern book on Lie groups, namely

"Structure and Geometry of Lie Groups" by Hilgert and Neeb.

It is a lovely book. It starts with matrix groups, develops them in great details, then goes on to do Lie algebras and then delves into abstract Lie Theory. Although they develop the requisite differentiable manifold theory in the text, I would also suggest

"An Introduction to Manifolds" by Loring W. Tu

for the manifolds part.

I also endorse

"Lie Groups, Lie Algebras, and Representations" by Brian C. Hall

for an elementary introduction to matrix Lie groups.

The last few sections of Teleman's representation theory notes are on the representation theory of the unitary group. I found them to be quite interesting, and a good introduction to Lie groups without Lie algebras. They won't get you to E_8, but they're still a good way to get into the subject if you already understand finite groups and their representations.

There have been a lot of terrific recommendations above(below?),but my favorite book on the subject hasn't been mentioned yet: Claudio Procesi's Lie Groups: An Approach through Invariants and Representations. Not only is it by one of the world's most respected researchers on the subject, it's probably the single most gentle book on the subject,even more so then Hall's book. The prerequisites are basically linear algebra and some rigorous calculus-everything else, including the concepts of differential manifolds, topology,tensor algebra and representation theory, are developed as needed in the book. It's very well written with a lot of strong exercises-to me,it's the best book for self study on the subject.

For students who don't have the patience to read through Procesi, there's a wonderful short chapter at the end of E. Vinberg's A Course In Algebra. It's gentle,builds on many concrete examples and gives the bare minimum students need to know.Also,as I've said many times before, I recommend Vinberg as probably my favorite single reference for algebra. Everyone serious about learning algebra should have a copy.

There is a nice book called Matrix Groups — An Introduction to Lie Group theory by Andrew Baker. It starts by talking on Matrix groups, then introduces Lie groups and shows that Matrix groups are in fact Lie groups. The last part is dedicated to the study of compact connected Lie groups. Note that it does not cover any representation theory. It is nice to read imho. A nice plus is that it repeats known results that are used. It really only assumes you have heard basic courses in linear algebra and analysis.

Another book that I actually liked a bit more is Naive Lie Theory from Stilwell. It is very easy to read and sometimes even a bit funny. It assumes only very, very basic mathematical knowledge and I recommend to read this to anyone who has never heard anything about that matter as a first read. I went through this book in two days, unable to put it down.

The book Lie Groups, Lie Algebras, and Representations – An Elementary Introduction from Brian Hall is a good book, as well. It doesn't read as good, but it seems to be nice as a reference book.

Another introductory book is Lie groups and algebras with applications to physics, geometry, and mechanics by Sattinger and Weaver.

An Introduction to the Lie Theory of One-Parameter Groups
https://www.forgottenbooks.com/.../AnIntroductiontotheLieTheoryofOneParameterGr...
Author: Abraham Cohen; Category: Calculus; Length: 256 Pages; Year: 1911.